The Ackerman steering principle states that when a vehicle with front steerable wheels negotiates a turn, the center of the turn lies on a line which is the extension of the rear axle center line. The theory of this principle is that, at least at low speeds, all tires rotate around this turn axis in a circle without slippage. The radius of this circle is determined by the steer angle of the two front steerable wheels. For proper steering geometry in a turn, the rotational axes of the front outside wheel and the front inside wheel should intersect at a common point. This point should be on the same turn axis of vehicle rotation. This theory is correct when the slip angles of all tires are zero, and is also thought to be a good design for steering mechanisms in general. To comply with Ackerman geometry the inside front wheel must have a greater steer angle than the outside front wheel. The difference in the inner wheel steer angle compared with the outside wheel steer angle is a function of the type of vehicle, suspension system and the design of its steering devices. Ackerman compliance will result in a pure rolling motion of the wheels, which is conducive to easier turning, less tire scuffing and less load reactions on the steering and suspension systems. The turn is defined by an axis perpendicular to the rear axle center line at a point which is the center of the turn.
The Ackerman theory first appeared in U.S. Pat. No. 663,986 issued in 1900. Several of the other referenced patents describe the Ackerman principle in detail, but describe mechanisms and detailed steering components, differing from one another in varying degrees, which improve steering and handling of the vehicle.
The theory of Ackerman extend to a four wheel steering vehicle suggests that minimum slippage of all tires will occur when they are rotating in a circle around a single turn axis. Therefore, the projection of rotational axes of all wheels should intersect this turn axis. One objective of U.S. Pat. No. 4498554, issued in 1985, was to achieve true Ackerman steering in the four wheel steered vehicle.
As described by R. B. Kazmier in his 1962 SAE paper entitled "Use of Computers in Steering Geometry Analysis", a given steering linkage system gives perfect steering for only one turning angle of the wheels. For other turn angles an error is introduced, the magnitude of which will depend upon the lengths and inclinations of the steering arms. Static toe and camber adjustments in the linkage are usually provided for the straight ahead position of the wheels or zero steer angles. A proper combination of steering linkages is necessary to minimize this error throughout the steering range.
One of the joint inventors in his 1986 SAE paper entitled "The Effect of Ackerman Steering Correction Upon Front Tire Wear of Medium Duty Trucks" gives data for actual tire tests which project 36.7% and 30.2% improvements in tire mileage with zero percent Ackerman error.
Having a correct steer angle for pure rolling motion is further influenced by the vehicle suspension during braking, acceleration, load transfer in a turn and a bump or depression in the surface. When the individual wheel of most vehicle suspension systems is extended (rebound) or compressed (jounce) the steer angle changes. The standard steering linkage system will not adjust for these changes. The object of U.S. Pat. No. 4796720, issued in 1989, is to provide a complete steering and suspension system which steers and tracks all wheels and provides a power assist for wheel camber adjustment and vehicle body leveling.
A variety of apparatus and systems exist which permits the sensing of the toe angle relative to the vehicle centerline. With known geometry of the steering and suspension system and the positions of one element in each linkage systems relative to the vehicle, the toe angles can be calculated. One such apparatus is described in U.S. Pat. No. 4,402,603, issued in 1983.